RF filter circuits may be used in wireless communication devices, e.g., mobile phones, to select wanted signals from a plurality of signals propagating in a plurality of frequency bands.
An important aspect of an RF filter circuit is the possible attenuation, i.e., the insertion loss that should be small in a pass band and high in a stop band. if the filter circuit is, in particular, used in pass band filters important aspects are bandwidth and the slope of the pass band flanks. If two pass band filters are combined, a duplexer can be obtained. An important parameter of duplexers is their isolation. The isolation mainly is the ratio of power fed into the transmission (TX) port divided by the power leaking to the reception (RX) port. Good attenuation (i.e., a high insertion loss) in a stop band—usually in the RX frequency band—is needed to obtain a good isolation.
In principle, filter circuits working with acoustic waves provide good attenuation. Filters with such filter circuits may be SAW filters (SAW=surface acoustic wave), BAW filters (BAW=bulk acoustic wave) or GBAW filters (GBAW=guided bulk acoustic wave). An electroacoustic filter is known from U.S. Pat. No. 6,441,704.
However, the ongoing trend towards miniaturization on the one hand, and more and more functionality on the other hand, jeopardizes the intrinsic good filter properties of such electroacoustic filters.
Thus, the need for filters with good filter properties even at small lateral dimensions exits. Further, duplexers comprising such filters are also needed.